Isotropic Source Research Articles

Subtle changes in strain prior to sub-Plinian eruptions recorded by vault-housed extensometers during the 2011 activity at Shinmoe-dake, Kirishima volcano, Japan

This study focuses on strain change observations with a precision of 10 −9 associated with the 2011 Shinmoedake eruptions in Japan, using vault-housed extensometers installed approximately 18 km northwest of the Shinmoe-dake crater. The extensometers recorded major strain changes of 10 −7 during three sub-Plinian eruptions and subsequent magma effusion. Our analysis indicates that these extensometer records provide a reasonable estimation of the parameters of an isotropic point source that can explain eruption-related ground deformation. The extensometers also recorded subtle strain changes of 10 −9 prior to the three sub-Plinian eruptions. Time series data indicate that changes in strain at these rates are generally only observed immediately before explosive eruptions, suggesting that these strain changes are precursors to sub-Plinian eruptions. The source of these subtle strain changes is likely to be shallower than the magma chamber associated with these eruptions. The precursory strain changes might have been caused by a pressure increase and a subsequent pressure decrease under the volcano. One possible scenario that can explain these pressure changes is the accumulation of volcanic gases at depth, causing an increase in pressure that was eventually released during gas emissions from the crater prior to the explosive eruptions.

One-Shot Lossy Quantum Data Compression

We provide a framework for one-shot quantum rate distortion coding, in which the goal is to determine the minimum number of qubits required to compress quantum information as a function of the probability that the distortion incurred upon decompression exceeds some specified level. We obtain a one-shot characterization of the minimum qubit compression size for an entanglement-assisted quantum rate-distortion code in terms of the smooth max-information, a quantity previously employed in the one-shot quantum reverse Shannon theorem. Next, we show how this characterization converges to the known expression for the entanglement-assisted quantum rate distortion function for asymptotically many copies of a memoryless quantum information source. Finally, we give a tight, finite blocklength characterization for the entanglement-assisted minimum qubit compression size of a memoryless isotropic qubit source subject to an average symbolwise distortion constraint.

Geometry-Based Spatial Sound Acquisition Using Distributed Microphone Arrays

Traditional spatial sound acquisition aims at capturing a sound field with multiple microphones such that at the reproduction side a listener can perceive the sound image as it was at the recording location. Standard techniques for spatial sound acquisition usually use spaced omnidirectional microphones or coincident directional microphones. Alternatively, microphone arrays and spatial filters can be used to capture the sound field. From a geometric point of view, the perspective of the sound field is fixed when using such techniques. In this paper, a geometry-based spatial sound acquisition technique is proposed to compute virtual microphone signals that manifest a different perspective of the sound field. The proposed technique uses a parametric sound field model that is formulated in the time-frequency domain. It is assumed that each time-frequency instant of a microphone signal can be decomposed into one direct and one diffuse sound component. It is further assumed that the direct component is the response of a single isotropic point-like source (IPLS) of which the position is estimated for each time-frequency instant using distributed microphone arrays. Given the sound components and the position of the IPLS, it is possible to synthesize a signal that corresponds to a virtual microphone at an arbitrary position and with an arbitrary pick-up pattern.

Propagation of a stochastic electromagnetic vortex beam in the oceanic turbulence

Abstract By using the extended Huygens–Fresnel principle, we investigate the stochastic electromagnetic vortex beam propagating through oceanic turbulence. General formulas for the elements of the 2×2 cross-spectral density matrix of a stochastic electromagnetic vortex beam propagating through the oceanic turbulence are obtained. We study the changes in the spectral density, the spectral degree of coherence and the spectral degree of polarization of such a vortex beam with the help of the general formulas. It is shown by numerical calculations that, the beam profile will approach a Gaussian distribution in far field under the influence of oceanic turbulence. It is also interesting to find that the spectral degree of polarization of a stochastic electromagnetic vortex beam composed by isotropic sources on propagation in far zone will return to its value in the source plane.

The angular distribution of energetic electron and X‐ray emissions from triggered lightning leaders

We investigate individual X‐ray bursts from lightning leaders to determine if energetic electrons at the source (and hence X‐rays) are emitted isotropically or with some degree of anisotropy. This study was motivated by the work of Saleh et al. (2009), which found the falloff of X‐rays in concentric radial annuli, covering all azimuthal directions in each annulus, from the lightning channel to be most consistent with an isotropic electron source. Here we perform a statistical analysis of angular and spatial distributions of X‐rays measured by up to 21 NaI/PMT detectors at the International Center for Lightning Research and Testing site for 21 leader X‐ray bursts from five leaders (including four dart‐stepped leaders and one dart leader). Two procedures were used to complete this analysis. Procedure 1 found the first‐order anisotropy, and procedure 2 tested whether or not the angular distribution was consistent with an isotropic distribution. Because higher‐order anisotropies could be present in the data, a distribution that is not isotropic does not necessarily have a significant first‐order anisotropy. Using these procedures, we find that at least 11 out of 21 X‐ray bursts have a statistically significant first‐order anisotropy, and hence those 11 are inconsistent with an isotropic emission. The remaining 10 bursts do not have significant first‐order anisotropy. However, of those 10 bursts, 9 are inconsistent with isotropic emission, since they exhibit significant higher‐order anisotropies. Since Saleh et al. (2009) did not consider anisotropies in the azimuthal direction, these new measurements of anisotropy do not necessarily contradict that work. Indeed, our analysis supports the finding that the X‐ray emissions from lightning are inconsistent with a vertically downward beam. The level of anisotropy of the runaway electrons is important because it provides, in principle, information on the streamer zone in front of the leader and the electric field near the lightning leader tip.

Exact analytical solution of time-independent neutron transport equation, and its applications to systems with a point source

An exact analytical solution of the time-independent monoenergetic neutron transport equation is obtained in this paper. The solution is applied to systems with a point source. Systematic analysis of the solution of the time-independent neutron transport equation, and its applications represent the primary goal of this paper. To the best of the author’s knowledge, certain key results on the scalar neutron flux as well as their derivations are new. As an application of these results, a scalar neutron flux for a purely absorbing medium with a spherically piecewise constant cross section and an isotropic point neutron source off the origin as well as that for a cylindrically piecewise constant cross section with a point neutron source off the origin are obtained. Both of these results are believed to be new.

Monte Carlo simulation of light scattering in the atmosphere and effect of atmospheric aerosols on the point spread function

We present a Monte Carlo simulation for the scattering of light in the case of an isotropic light source. The scattering phase functions are studied particularly in detail to understand how they can affect the multiple light scattering in the atmosphere. We show that, although aerosols are usually in lower density than molecules in the atmosphere, they can have a non-negligible effect on the atmospheric point spread function. This effect is especially expected for ground-based detectors when large aerosols are present in the atmosphere.

Evolution of the Spatial Parameters of a Light Pulse During Its Propagation in a Medium with Strongly Anisotropic Scattering

We obtain the equations for determining the spatial moments of the radial distribution of brightness in the pulsed radiation field of a point isotropic source and similar characteristics of the distribution of irradiance from a point unidirectional source on the assumption of strong anisotropy of the scattering indicatrix of a medium and the studied light field. The procedure of analytical solution of these equations is shown. Formulas for calculating the spatial parameters of the light pulse at a given time instant, i.e., the distance between the front edge and the barycenter of the pulse, as well as its characteristic length, are derived. It is shown how and under which conditions the time parameters of the pulse can be determined from its spatial parameters.

Experimental Verification of Isotropic Radiation from a Coherent Dipole Source via Electric-Field-DrivenLCResonator Metamaterials

It has long been conjectured that isotropic radiation by a simple coherent source is impossible due to changes in polarization. Though hypothetical, the isotropic source is usually taken as the reference for determining a radiator's gain and directivity. Here, we demonstrate both theoretically and experimentally that an isotropic radiator can be made of a simple and finite source surrounded by electric-field-driven LC resonator metamaterials designed by space manipulation. As a proof-of-concept demonstration, we show the first isotropic source with omnidirectional radiation from a dipole source (applicable to all distributed sources), which can open up several possibilities in axion electrodynamics, optical illusion, novel transformation-optic devices, wireless communication, and antenna engineering. Owing to the electric- field-driven LC resonator realization scheme, this principle can be readily applied to higher frequency regimes where magnetism is usually not present.

Rigorous Asymptotic and Moment-Preserving Diffusion Approximations for Generalized Linear Boltzmann Transport in Arbitrary Dimension

We derive new diffusion solutions to the monoenergetic generalized linear Boltzmann transport equation for the stationary collision density and scalar flux about an isotropic point source in an infinite d-dimensional absorbing medium with isotropic scattering. We consider both classical transport theory with exponentially distributed free paths in arbitrary dimensions as well as a number of nonclassical transport theories (nonexponential random flights) that describe a broader class of transport processes within partially correlated random media. New rigorous asymptotic diffusion approximations are derived where possible. We also generalize Grosjean’s moment-preserving approach of separating the first (or uncollided) distribution from the collided portion and approximating only the latter using diffusion. We find that for any spatial dimension and for many free-path distributions Grosjean’s approach produces compact, analytic approximations that are, overall, more accurate for high absorption and for small source-detector separations than either P1 diffusion or rigorous asymptotic diffusion. These diffusion-based approximations are exact in the first two even spatial moments, which we derive explicitly for various nonclassical transport types. We also discuss connections between the random-flight-theory derivation of the Green’s function and the discrete spectrum of the transport operator and report some new observations regarding the discrete eigenvalues of the transport operator for general dimensions and free-path distributions.

Correcting for the barrier effect of protection when using photon buildup factors

ditions due to sources of photon radiation. The main source of errors in calculations of the dose characteristics performed with these methods is incorrectly taking account of the contribution of the scattered photon radiation. Ordinarily, the scattered photon radiation is taken into account by means of dose buildup factors obtained by the method of moments in infinite media with atomic number Z and distance μd between a point isotropic source of photons with energy E0 and a detector – B∞(E0, μd, Z) [1, 2]. However, in most cases the protection must be regarded at least as a barrier, and then it is necessary to know the buildup factors Bbar(E0, μd, Z) for the barrier geometry of the protection. The barrier effect of the protection is usually taken into account by introducing a correction δ. There are many methods for taking this effect into account [1]. It is desirable to represent the correction using a method in which it does not depend on the thickness of the protection. This method was proposed for dose buildup factors [1], where the correction was introduced in the form

Response Function of Collimated Detector for Non Axial Detector-Source Geometry

Response function is a fundamental parameter for all detectors in order to analyze the energy distribution of gamma ray which undergoes scattering interaction with the material. The response functions of a 3 in. x 3 in. NaI(Tl) collimated detector for non axis detector-source geometry has been calculated using a Monte Carlo approach from GEANT4 simulation code with 0.662 MeV of mono-energetic of photon gamma ray. Collimated Pb with 4 cm thickness and 2 cm of holes diameter were employed for shielding. The source was assumed as an isotropic point source and it is placed at various positions to the detector axis. The comparison between the measured energy response functions and the simulated energy response functions after normalization were also performed in order to validate the modeling results.

Quantum Rate-Distortion Coding With Auxiliary Resources

We extend quantum rate distortion theory by considering auxiliary resources that might be available to a sender and receiver performing lossy quantum data compression. The first setting we consider is that of quantum rate distortion coding with the help of a classical side channel. Our result here is that the regularized entanglement of formation characterizes the quantum rate distortion function, extending earlier work of Devetak and Berger. We also combine this bound with the entanglement-assisted bound from our prior work to obtain the best known bounds on the quantum rate distortion function for an isotropic qubit source. The second setting we consider is that of quantum rate distortion coding with quantum side information (QSI) available to the receiver. In order to prove results in this setting, we first state and prove a quantum reverse Shannon theorem with QSI (for tensor-power states), which extends the known tensor-power quantum reverse Shannon theorem. The achievability part of this theorem relies on the quantum state redistribution protocol, while the converse relies on the fact that the protocol can cause only a negligible disturbance to the joint state of the reference and the receiver's QSI. This quantum reverse Shannon theorem with QSI naturally leads to quantum rate-distortion theorems with QSI, with or without entanglement assistance.

Calculation of gamma buildup factors for point sources

Objective of this study is to calculate gamma buildup factors for pointed and isotropic gamma sources in depleted uranium, uranium dioxide, natural uranium, tin, water and concrete using MCNP4C code. The thickness of the media ranges from 0.5 to 10 mean-free-path (mfp) and gamma energy ranges from 0.5 to 10 MeV. Owing to the outstanding accuracy of MCNP in calculation involving gamma interaction, results fairly match those reported previously. The maximum relative error is 2%.

Theoretical and numerical results on effects of attenuation on correlation functions of ambient seismic noise

S U M M A R Y We study analytically and numerically effects of attenuation on cross-correlation functions of ambient noise in a 2-D model with different attenuation constants between and outside a pair of stations. The attenuation is accounted for by quality factor Q(ω) and complex phase velocity. The analytical results are derived for isotropic far-field source distribution assuming the Fresnel approximation and mild attenuation. More general situations including cases with non-isotropic source distributions are examined with numerical simulations. The results show that homogeneous attenuation in the interstation regions produces symmetric amplitude decay of the causal and anticausal parts of the noise cross-correlation function. The attenuation between the receivers and far-field sources generates symmetric exponential amplitude decay and may also cause asymmetric reduction of the causal/anticausal parts that increases with frequency. This frequency dependence can be used to distinguish asymmetric amplitudes due to attenuation from frequency-independent asymmetry in noise correlations generated by nonisotropic source distribution. The attenuations both between and outside station pairs also produce phase shifts that could affect measurements of group and phase velocities. In terms of noise cross-spectra, the interstation attenuation is governed by Struve functions while the attenuation between the far-field sources and receivers is associatedwith exponential decay and the imaginary part of complex Bessel function. These results are fundamentally different from previous studies of attenuated coherency that append the Bessel function with an exponential decay that depends on the interstation distance.

WE‐A‐108‐07: Dynamic Modulated Brachytherapy for Accelerated Partial Breast Irradiation

Purpose: To develop a novel brachytherapy applicator for HDR‐based accelerated partial breast irradiation (APBI) that significantly improves upon those commercially available, thereby reducing acute and late side effects seen with current APBI techniques. Methods: First, the limitations of adjusting the channel placements or placing more channels in a balloon‐like multi‐catheter ABPI device were explored. Then, a novel tungsten‐shielded applicator design was evaluated. The applicator design consists of a 9‐mm diameter tungsten rod, density=19.0gm/cm3, with 8 sunken grooves along the edge to allow passage of the 192Ir source. Using dose distributions calculated with Monte Carlo (MCNPX), optimized plans were simulated for our devices, as well as the MammoSite and SAVI, for a set of realistic phantom geometries. These geometries, based on patient cases, were ellipsoids of varying sizes with different PTV orientations and a surrounding rind of healthy tissue to be spared. Results: The benefits of adding more channels become insignificant after about 7 channels. Calculations were done for up to 361 channels, but it was found that this was insufficient to create any noticeable improvement. However, the novel shielded design significantly decreased the V150 and V200, while maintaining the target coverage (V90). All planning variables were superior for this design, except for a modest increase in the total dwell time. Conclusions: The introduction of additional channels did little to aid the performance of a balloon‐like design. There was a very slight benefit to the dose distributions, but not nearly enough to warrant further investigations. Based on these findings, any device based on open isotropic radiation source modulation cannot be improved much further. Therefore, it is necessary to consider designs with directional radiation profiles, which we have shown to give superior coverage. Our new tungsten shielded applicators are a large step forward along the path of brachytherapy applicator development.

SU‐E‐T‐32: Monte Carlo Determination of WAFAC Corrections for the Canadian LDR Primary Standard

Purpose: Determine correction factors for a wide angle free‐air chamber (WAFAC) using Monte Carlo techniques for the realization of the air‐kerma strength, Sk, for low dose rate (LDR) 125 I and 103Pd brachytherapy sources. This WAFAC will become the Canadian primary standard for LDR brachytherapy sources. Methods: The EGSnrc user‐code egsfac is used for estimating the correction factors required to determine the air‐kerma at the WAFACs point of measurement (POM) using MC simulation for a 125 I and a 103Pd source. The geometry library egs++ allows simulation of the sources and the WAFAC in detail. Spectra are taken from the most recently published data. Air and aluminum attenuation curves for the 125 I seed were also measured with the WAFAC. Results: Correction values compared with those reported by NIST for its LDR primary standard (of very similar design) are in excellent agreement when using the NIST approach of assuming an isotropic point source and using experimental main spectrum lines. Differences in both methodologies for determining the WAFAC corrections are discussed and their impact evaluated. The effect of the source and the holder on the scatter correction is quantified separately. The calculations resulted in a modification of the design to reduce collimator transmission. A 3.2 mm lead sheet was added behind the WAFAC aperture, attenuating the 1251 and the 103Pd photon beams to less than 0.02 %. MC attenuation curves for air and aluminum for the 125 I seed are in good agreement with experimental data. The largest correction is the aluminum filter attenuation which can be as large as 4 %. Conclusion: Despite an inconsistency in the scatter estimation, the NIST approximation has little impact on the determination of Sk. The scatter correction is sensitive to the collimation system dimensions. Using MC‐based air and aluminum attenuation corrections for the 125 I seed is experimentally justified.

Spatio‐temporal variations of double‐couple aftershock mechanisms and possible volumetric earthquake strain

AbstractWe analyze spatio‐temporal patterns in rotation angles of double‐couple–constrained mechanisms of aftershocks of the 1992 Landers earthquake. The rotation angles provide information on the distribution of source geometries in different regions of space and time with respect to the mainshock focal mechanism. The results indicate that the mechanisms of the early aftershocks are more scattered and less aligned with the mainshock than those of the long‐term events. This is most pronounced around the northern end of the Landers rupture, least pronounced around the central section, and intermediate around the southern end of the rupture. The relatively large scatter and misalignment of the mean rotation angles of the early focal mechanisms around the edges of the Landers rupture suggest possible volumetric earthquake strain in these regions. The results may reflect isotropic source terms produced by dynamic generation of rock damage. Synthetic tests indicate that the observed differences in the rotation distributions of the early and long‐term events around the end regions of the Landers rupture can result from neglecting in the inversion process isotropic components that are 0.03–0.15 of the total event moments.

Detection of an Absorbing Heterogeneity in a Biological Object during Recording of Scattered Photons

A rapid method for detection of an optical heterogeneity in a biological object on the basis of diffuse optical tomography (DOT) is described. The method utilizes late arriving photons (LAP) that are scattered and diffusely transmitted through a phantom or a biological object. The technique was validated experimentally using a near infrared femtosecond laser as the source. A streak camera was used as the radiation detector. Satisfactory agreement between the experimental data and a numerical diffusion model is demonstrated. The initial relative number of photons in a single pulse (virtual isotropic source) uniformly fills the object in the same manner as a diffusing droplet moving toward the center of an object.

Optical power and energy radiated by natural lightning

AbstractCalibrated measurements of the visible and near‐infrared radiation produced by both negative and positive cloud‐to‐ground (CG) lightning strokes have been made at distances of 5 to 32 km in southern Arizona (AZ) and the central Great Plains using a photodiode sensor with a flat spectral response between 0.4 and 1.0 µm. Time‐correlated video images (60 fps) of the channel development provided information about the types of strokes that were detected and reports from the U.S. National Lightning Detection Network indicated their locations, polarities, and estimates of their peak current. In our sample of negative strokes that were suitable for analysis, there were 23 first (or only) strokes (FS), 19 subsequent strokes that created new ground contacts (NGC), and 101 subsequent strokes that re‐illuminated a preexisting channel (PEC). We also analyzed 10 positive strokes (in nine flashes), and 73 of the larger impulses that were radiated by intracloud discharges (CPs). Assuming that these events can be approximated as isotropic sources and that the effects of atmospheric extinction are negligible, the peak optical power (Po), total optical energy (Eo), and characteristic widths of the sources (tcw = Eo/Po) have been computed. Median values of Po for negative FS, NGC, and PEC strokes were 1.8 × 1010 W, 1.1 × 1010 W, and 4.4 × 109 W, respectively. Median values of Eo were 3.6 × 106 J, 3.5 × 106 J, and 1.2 × 106 J, respectively. The median characteristic widths of negative FS, NGC, and PEC strokes were 229 µs, 244 µs, and 283 µs, respectively. Positive CG strokes produced a median Po, Eo, and tcw of 1.9 × 1010 W, 9.3 × 106 J, and 497 µs, respectively. Estimates of the space‐and‐time‐average power per unit length () in the lower portion of negative FS, NGC, and PEC channels had medians of 2.8 × 106 W/m, 3.2 × 106 W/m, and 1.4 × 106 W/m, respectively, and the median for four positive strokes was 8.8 × 106 W/m. Median values for the estimated peak electromagnetic power (PEM) radiated at early times in the strokes are 2.0 × 109 W, 2.5 × 109 W, 1.0 × 109 W, and 9.1 × 109 W for FS, NGC, PEC and positive strokes, respectively. CP events produced a median Po, Eo, and tcw of 2.0 × 109 W, 0.7 × 106 J, and 311 µs, respectively, and are in good agreement with aircraft and satellite measurements. The values of Po, Eo, and for negative CG strokes in AZ are significantly larger than prior measurements in Florida, likely because there is less atmospheric extinction in our dataset, and due to extinction, all the above values of Po, Eo, and are lower limits at the source.